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Indian Journal of Chemistry
Vol. 54A, July 2015, pp. 867-871
Notes
Reversible hydration of
tetraaquabis(4-aminobenzoato)cobalt(II)
Kiran T Dhavskar & Bikshandarkoil R Srinivasan*
Department of Chemistry, Goa University, Goa 403206, India
Email: [email protected]
dihydrate can be reversibely hydrated8. In this work
we have chosen the known zero dimensional cobalt
complex, namely tetraaquabis(4-aminobenzoato)cobalt(II)
(1), which contains only coordinated but no lattice
water molecules9 for investigating changes in
the property on the dehydration and rehydration.
The results are described herein.
Received 22 May 2015; accepted 30 June 2015
The
octahedral
red
complex
tetraaquabis(4-aminobenzoato)cobalt(II) (1) exhibits a water induced crystallineamorphous-crystalline
transformation
accompanied
by
chromotropism. The loss of coordinated water from
[Co(4-aba)2(H2O)4] (1) (4-aba=4-aminobenzoate) on heating,
leads to the formation of tetrahedral anhydrous [Co(4-aba)2] (2).
The amorphous blue compound (2) can be rehydrated to original
composition by equilibrating (2) over water vapour. The
rehydrated crystalline compound exhibits properties identical
with (1). The variation in the structure and properties
of (1) due to the coordination and decoordination of water
are studied by spectral, thermal, magnetic and conductivity
measurements.
Keywords: Chromotropism, Crystal-to-amorphous transformations,
Dehydration, Rehydration, Reversible hydration,
Cobalt, 4-Aminobenzoate
Progress and development of science and technology
relies mostly on the possibility of obtaining various
materials with properties designed to meet practical
purposes. This possibility is mainly based on the
correlation between structure and properties of
materials. Thus careful studies of material properties
and correlating the corresponding structural changes
can serve as a lead in exploring and devising synthetic
methodologies that can help to design materials of
potential applications. Several publications have
appeared in recent literature, showing the growing
research interest in metal-organic materials with
flexible and dynamic frameworks especially in those
that reversibly change their structures and properties
in response to external stimuli as they have many
applications1-5. These types of changes especially the
ones accompanied by loss of coordinated or lattice
water have also been reported for zero-dimensional
systems for example hydrated organic ammonium
salts6,7 or tetra(aqua)Co(II) complexes8. Cobalt is a
typical metal ion that shows chromism upon ligand
exchange in the solid state. In an earlier work we have
shown that tetraaquabis(paranitrobenzoato)cobalt(II)
Experimental
All the chemicals used in this study were of
reagent grade and all the syntheses were carried out
using doubly distilled water. Infrared (IR) spectra
were recorded on a Shimadzu (IR Prestige-21)
FT-IR spectrometer in the range 4000–400 cm-1.
UV-vis diffuse-reflectance spectra were recorded on a
Shimadzu UV-2450 double beam spectrophotometer.
BaSO4 powder was used as reference (100%
reflectance). Absorption data were calculated from the
reflectance data using the Kubelka-Munk function
(a/S=(1-R2/2R where a is the absorption coefficient,
R the reflectance and S the scattering coefficient).
TG-DTA studies were carried out on Netzsch, STA
409 PC (Luxx) analyzer, from RT to 1000 °C
in dry air, with a heating rate of 10 K min-1.
Isothermal weight loss studies were performed in an
electric furnace fitted with a temperature controller
and on a steam bath. The X-ray powder pattern were
recorded using Cu-Kα radiations of λ= 1.5418 Ǻ
(filtered through Ni) in steps of 0.02 degrees on a
RIGAKU Ultima IV diffractometer. The variation of
electrical resistivity, as a function of temperature was
measured using two probe electrical conductivity
setup. The temperature variation of resistance was
measured from room temperature to 350 oC using a
Keithley electrometer. The magnetic susceptibility in
air was determined by Guoy method at room
temperature in a field of 9600 gauss using sensitive
analytical balance. A Quantum Design PPMS-VSM
magnetometer was used for magnetic characterization
of the pelletized compounds. The variation of the
dc-susceptibility of each sample with temperature
was measured from 2 K to 300 K in the ZFC (zero
field cooling) and FC (field cooling) modes using a
magnetic field of 500 Oe. The magnetization with
varying magnetic field of up to 140 kOe was also
measured at 2 K and 100 K.
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INDIAN J CHEM, SEC A, JULY 2015
For the synthesis of tetraaquabis(4-aminobenzoato)
cobalt (II) (1), cobaltous carbonate was freshly
prepared by mixing a solution of Na2CO3 (1.06 g,
10 mmol) in water (10 mL) with [Co(H2O)6]Cl2
(2.37 g, 10 mmol) in water (10 mL). To this,
4-aminobenzoic acid (2.74 g, 20 mmol) in water
(50 mL) was added and the reaction mixture was
heated on a water bath.. The reaction was complete
within 30 min as compared to commercial CoCO3,
which requires a prolonged reaction time. The
reaction mixture containing a very small amount of
insoluble material was filtered and the filtrate was left
undisturbed for crystallization. After 4-5 days, red
coloured crystals (1) were obtained (yield: 78%).
Alternatively, these crystals can also be isolated in
85% yield from a solution containing the sodium salt
of 4-aminobenzoic acid and cobalt chloride in 1:2
mole ratio.
To study the formation and rehydration of
[Co(4-aba)2] (2), a powdered sample of (1) (500 mg)
was taken in a silica crucible and heated in an oven
for ~ 15 min at 140 oC. This resulted in the formation
of dark blue coloured complex (2) (408 mg).
The crucible with complex (2) was equilibrated over
water in a dessicator. After a day, the colour of the
complex changed from dark blue to the original red,
resulting in isolation of the rehydrated compound.
Results and discussion
Aqueous reaction of CoCO3 (Eq. 1) with
4-aminobenzoic acid (4-abaH) in a 1:2 mole ratio
results in the formation of a hexacoordinate
red compound [Co(4-aba)2(H2O)4] (1). Freshly
precipitated CoCO3 is solubilised by hot aqueous
4-abaH in ~30 min while commercial CoCO3 requires
a longer reaction time. The reaction of [Co(H2O)6]Cl2
with in situ generated sodium salt of 4-aminobenzoic
acid can also be used as an alternate method (Eq. 2)
for compound synthesis.
CoCO3+ 2(4-abaH) →
[Co(4-aba)2(H2O)4] (1) + H2O + CO2↑
which is in agreement with literature9. The single
crystal work reveals that in (1), the central metal
is six coordinated and 4-aba anion functions as a
monodentate carboxylate ligand. The central metal
ion, i. e., Co(II) is surrounded by six oxygen atoms,
four of which are from aqua ligands forming a
square plane and the two monodentate carboxylate
ligands disposed trans to each other complete
the
octahedral
coordination
around
cobalt
(Supplementary data, Fig. S1).
During the dehydration and rehydration studies, on
heating at 140 °C the red coloured (1) changes to the
anhydrous blue compound (2), which regains its
original colour on equilibrating over water. The
colour changes can be explained as dehydration of (1)
resulting in the formation of blue anhydrous
compound (2) as evidenced by the loss in mass
accompanied by a characteristic IR spectrum. It is
interesting to note that dehydration results in loss of
crystallinity of (1) as evidenced by the X-ray powder
pattern (Fig. 1). Although the anhydrous compound
(2) is amorphous, both (1) and the rehydrated
compound (3) are crystalline, showing that
rehydration results in the transformation of the
amorphous material to a crystalline solid. In the
rehydration process, on exposure to moisture the
dehydrated form of (1) attains its original colour. The
rehydrated compound labeled as (3) is actually
compound (1) which can be inferred by comparison
of its spectral, thermal and conductivity
characteristics with that of (1). In view of the
quantitative formation of the rehydrated compound,
the entire process of dehydration and rehydration can
be schematized as shown below.
... (1)
[Co(H2O)6]Cl2 + 2{ NaHCO3 + (4-abaH) } →
[Co(4-aba)2(H2O)4] + 2NaCl ... (2)
The product was unambiguously characterized
with the aid of single crystal X-ray crystallography
and confirmed to be the known tetraaquabis
(4-aminobenzoato)cobalt(II); the compound was
formulated as [Co(4-aba)2(H2O)4] (1) based on the
unit cell data (Supplementary data, Tables S1-S3),
Fig. 1 – X-ray powder patterns of the red [Co(H2O)4(4-aba)2] (1),
anhydrous blue [Co(4-aba)2] (2) and the rehydrated compound (3).
NOTES
∆
[Co(4 − aba ) 2 (H 2 O) 4 ]
→
←
[Co(4 - aba) 2
Rehydration
The coinciding of the IR spectra of (1) and the
rehydrated compound (3) serve to demonstrate that
they are one and the same (Fig. 2). The free acid
4-abaH does not show broad band above 3100 cm-1
unlike the hydrated compound [Co(4-aba)2(H2O)4],
indicating the presence of water in (1).
The hydrated and rehydrated complexes exhibit
very strong and broad absorption bands in the region
3500–3000 cm-1, corresponding to the asymmetric
and symmetric stretching vibrations of coordinated
water which appears broad due to extensive
intermolecular hydrogen bonding10,11. Further the
ν(Μ−Ο) at 472 cm-1 and 555 cm-1 are indicative of
coordinated aqua complex and the band at 640 cm-1
may be due to wagging mode of coordinated water11.
The IR spectrum of (1) heated at 140 °C shows
several IR bands while the broad signal due to water
disappears. The absence of the broad O-H band in the
heat-treated sample can be attributed to the formation
of the corresponding anhydrous complex. The two
signals at 3354 and 3221 cm-1 of medium intensity
(Supplementary data, Fig. S2) clearly visible in
anhydrous complex (2) and are masked by the
broad O-H absorption signal in (1) and rehydrated
complex can be assigned to the N-H asymmetric
and symmetric stretching vibrations as seen for a
primary amine10. The absence of bands ν(Μ−Ο) at
472 cm-1, 555 cm-1 and ρw(H2O) at 640 cm-1 bands
(Supplementary data, Fig. S3) further supports the
anhydrous nature. In view of this, the band at 502 cm-1
can be assigned to ν(Co-O) where oxygen is from
Fig. 2 – Infrared spectra of the red [Co(H2O)4(4-aba)2] (1),
anhydrous blue [Co(4-aba)2] (2) and the rehydrated
compound (3).
869
carboxylate. Τhe bands (Supplementary data, Fig. S4)
at 1528 cm-1 and 1379 cm-1 are assigned as
asymmetric and symmetric stretch of carboxylate in
(1) and rehydrated compound. The binding modes
of metal ions to the carboxylate ligands has been a
subject of interest and structural diagnosis using
carboxylate stretching frequencies have been
investigated in several studies12-17. In this study we
report the vibrational spectra and correlate the binding
modes of carboxylate ligand for both the crystalline
(1) and the amorphous (2). The vibrations of
the carboxylate group are closer in (2) (1539 cm-1
and 1393 cm-1) with a slightly smaller difference in
asymmetric and symmetric stretch than in (1).
The bands are split indicating that the carboxylate
is bidentate in the dehydrated compound11-17.
The blue colour of (2) which is characteristic of
the tetrahedral Co (II) also supports this assignment.
It is observed that the intensity of the asymmetric
stretch of carboxylate is slightly less than that of
the symmetric stretch in (1) and this intensity
difference increases in (2) (Fig. S4). Bidentate
chelation makes both C-O bonds symmetrical
and hence the C-O stretch initially seen in
hydrated complex and free acid disappears in
the dehydrated complex and the spectrum
exhibits symmetric and asymmetric vibrations of
carboxylate shifted to higher frequency region.
The band at 1177 cm-1 can be assigned to C-N stretch.
The rehydration of (2) resulted in the isolation of the
rehydrated compound (3).
Removal of coordinated water from Co(II) leads to
energy changes of the d orbitals of the central metal,
as evidenced by the characteristic blue colour of
tetrahedral Co(II).The forbidden d-d transitions in
octahedral complexes with a center of symmetry are
observed as weak bands due to vibrational motions
which break this symmetry. In tetrahedral complexes
with no center of symmetry the transitions are
relatively intense (Fig. 3). The data show very weak
d-d transition for (1) and (3) as expected
(Supplementary data, Fig. S5). Secondly since ∆t is
much lower than ∆o complexes of the same ion with
ligands
occupying
nearby
positions
in
spectrochemical series have their absorption maxima
for d-d spectra at longer wavelengths for tetrahedral
complexes. UV-DRS data show the absorption
maxima for complex (1) and (3) at 479 nm, 502 nm
and for complex (2) at 577 nm. Charge transfer bands
are seen for both complexes (1) and (2).
870
INDIAN J CHEM, SEC A, JULY 2015
The isothermal mass loss studies reveal a mass
loss of 12.35% on heating the tetraaqua compound
(1) at 100 oC, which corresponds to a loss of three
moles of water. This value is in agreement with
the theoretical mass loss of 13.39%. A second
mass loss (5.04%) occurs between 110–150 °C,
corresponding to the loss of one water molecule.
The total mass loss (17.39%) at this temperature
is in very good agreement with the calculated mass
loss of 17.85%, which corresponds to the total loss
of 4 moles of water resulting in the formation of blue
colored complex. The DTA curve of (1) exhibits an
endothermic peak at 118 oC which can be assigned
for the loss of coordinated water from the central
metal. The endothermic peak at 330 oC and
corresponding weight loss of 21.83% indicates the
loss of two moles of CO2. The decaboxylation of
Fig. 3 – UV-vis spectra of (1), (2) and rehydrated compound (3).
organic ligand is followed by an exothermic event at
421 °C which can be attributed to the decomposition
of the remaining organic moiety. The assignment of
the first thermal event for the dehydration process
can be confirmed by the absence of this signal in (2),
and the absence of the O-H vibration in the IR
spectrum of (2). No mass loss is observed for (2) in
this temperature range while the rehydrated complex
exhibits an identical weight loss profile as that of
(1). The identical nature of (1) and (3) can also be
evidenced from the TG-DTA graphs (Fig. 4a). It is
interesting to note that the thermal profiles of (1) and
(2) are identical after the first event clearly
indicating that the first step in the decomposition of
(1) is removal of coordinated water.
The different nature of (1) and (2) can be further
evidenced by a sudden decrease in DC resistivity
of the hydrated and rehydrated compounds (Fig. 5),
however no such change is observed for the
anhydrous blue compound which exhibits a straight
line parallel to the X-axis. This change is observed
between 110–130 oC for complex (1) and (3). This is
the same temperature range when the loss of last
coordinated water molecule causes change in the
coordination sphere of cobalt and hence resulting in
the structural rearrangement from octahedral to
tetrahedral with loss of crystalline nature.
A magnetic moment of 5.11 µB consistent with an
octahedral arrangement of the metal centre indicates a
high spin cobalt complex18, was measured for (1). The
magnetic moment of the anhydrous compound (2)
decreased to 4.01 µB. The M-T plots (Fig. 6 &
Fig. 4 – TG-DTA curves of tetraaquabis(4-aminobenzoato)cobalt(II). [Inset: TG-DTA profiles of [Co(H2O)4(4-aba)2] (1) [Co(4-aba)2]
(2) and the rehydrated compound (3)].
NOTES
871
hysteresis curves with zero remanance and coercivity at
2 K which is typical of a superparamagnetic material.
In this work it is shown that compound (1) undergoes
crystalline–amorphous-crystalline
transformation.
The six cordinated octahedral tetraaquabis(4-aminobenzoato)cobalt(II) (1) can be fully dehydrated to four
coordinated tetrahedral amorphous bis(4-aminobenzoato)cobalt(II) (2). Compound (2) can be
rehydrated to the starting material thereby regaining
its original properties.
Fig. 5 – DC resistivity plot of [Co(H2O)4(4-aba)2] (1),
[Co(4-aba)2] (2) and the rehydrated compound (3).
Supplementary data
Supplementary data associated with this article,
i.e., Figs S1-S8, and, Tables S1–S3, are available
in the electronic form at http://www.niscair.res.in/
jinfo/ijca/IJCA_54A(07)867-871_SupplData.pdf.
Acknowledgement
The authors thank Dr. Alok Banerjee, UGC-DAE
Consortium for Scientific Research, Indore, India, for
the magnetic data and helpful discussion.
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Fig. 6 – M versus T plot of [Co(H2O)4(4-aba)2] (1). [For the M
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